In recent years, because of high efficiency and high output of a variety of heat engines, a tendency toward an increase in burning temperature, exhaust gas temperature, or steam temperature has been increased, and in response thereto, a requirement for an enhancement of strength characteristics in heat-resistant steels has been also increased. As a heat-resistant steel to be used for the foregoing heat-resistant application, JIS SUH660, that is a γ′ precipitation type iron base superalloy, has hitherto been frequently used for the use at a temperature of up to 700° C. However, accompanied with high efficiency and high output of a variety of heat engines, there is a concern about a shortage of the strength. In addition, SUH660 involves such a problem that the precipitation of an η phase (Ni3Ti) is brought due to the use over a long period of time, resulting in lowering of the strength and ductility. Furthermore, SUH660 contains a large quantity of expensive Ni, so that it involves such a problem that the cost becomes high.
Incidentally, as the related-art technologies relative to the invention, those disclosed in the following Patent Documents 1 and 2 are exemplified.
Patent Document 1 discloses an invention regarding “heat-resistant bolts”. The invention disclosed in Patent Document 1 is aimed to obtain a heat-resistant bolt with excellent relaxation characteristics, in which by optimizing blending of chemical components and working method, even when cold working is applied, the precipitation of an η phase can be suppressed in a subsequent process at a high temperature under a high stress. However, Patent Document 1 does not mention the characteristic features of the present invention, i.e., an increase of an age-hardening amount after cold working by positively incorporating Mn; and an improvement of a balance between cold workability and high-temperature strength by specifying a total amount of Ni and Mn and a ratio thereof.
Patent Document 2 discloses an invention regarding “heat-resistant stainless steels”. The invention of Patent Document 2 is aimed to provide a heat-resistant high-strength stainless steel which is excellent in high-temperature tensile strength of spring in a high-temperature zone and high-temperature permanent set resistance by controlling the precipitation amount and form of each of a γ′ phase and an η phase. However, Patent Document 2 does not mention the characteristic features of the present invention, i.e., reduction of the Ni amount to achieve suppression of costs and at the same time, an improvement of a balance between cold workability and high-temperature strength, by specifying a total amount of Ni and Mn and a ratio thereof.    Patent Document 1: JP-A-2001-158943    Patent Document 2: JP-A-2000-109955